Instructions

for the custard

Combine vanilla and espresso powder in small bowl and set aside. Whisk sugar, cornstarch, cocoa, and salt together in large saucepan. Whisk in ½ cup milk and egg yolks until fully incorporated, making sure to scrape corners of saucepan. Whisk in remaining 2½ cups milk until incorporated.
Whisk gently over medium heat until mixture is thickened and bubbling over entire surface, 5 to 8 minutes. Cook 30 seconds longer, remove from heat, add chocolate and butter, and whisk until melted and incorporated. Whisk in vanilla mixture.
Pour custard through fine-mesh strainer into wide, shallow bowl. Press lightly greased parchment paper against surface of custard and refrigerate until cool, at least 4 hours or up to 2 days.

for the raspberry filling

Place half of raspberries, sugar, cornstarch, and salt in medium saucepan. Place remaining raspberries in large bowl. Using potato masher, thoroughly mash raspberries in saucepan. Cook over medium heat, stirring frequently, until sugar is dissolved and mixture is thick and bubbling, 3 to 5 minutes. Pour over raspberries in bowl and stir to combine. Set aside.

for the cake

Adjust oven rack to middle position and heat oven to 350 degrees. Lightly grease 18 by 13-inch rimmed baking sheet, line with parchment, and lightly grease parchment. Whisk flour, sugar, baking powder, and salt together in medium bowl. Whisk oil, water, egg yolks, and vanilla into flour mixture until smooth batter forms.
Using stand mixer fitted with whisk attachment, whip 5 egg whites and cream of tartar on medium-low speed until foamy, about 1 minute. Increase speed to medium-high and whip until soft peaks form, 2 to 3 minutes. Transfer one-third of whites to batter; whisk gently until mixture is lightened. Using rubber spatula, gently fold remaining whites into batter.
Pour batter into prepared sheet; spread evenly. Bake until cake springs back when pressed lightly in center, 10 to 13 minutes.
Transfer cake to wire rack; let cool for 5 minutes. Run knife around edge of sheet, then invert cake onto rack. Remove parchment, then re-invert cake onto second wire rack. Let cool completely, at least 30 minutes.

for the whipped cream

Using clean, dry mixer bowl and whisk attachment, whip 1 cup cream and 2 teaspoons sugar on medium-low speed until foamy, about 45 seconds. Increase speed to high and whip until soft peaks form, about 1 minute. Trim ¼ inch off each side of cake; discard trimmings. Cut cake into 24 equal pieces (each piece about 2½ inches square). Briefly whisk custard until smooth.
Spoon ½ cup raspberry mixture into trifle bowl and spread over bottom. Tear 1 cake square into 4 pieces and pile in center of bowl. Shingle 10 cake squares, fallen domino–style, around bottom of trifle, placing edges against bowl wall. Tear another cake square into 4 pieces and fill in center. Drizzle 3 tablespoons rum evenly over cake. Spoon half of custard over cake and spread evenly. Spoon whipped cream over custard and spread evenly. Spoon remaining raspberry mixture over cream and spread evenly.
Repeat layering with remaining 12 cake squares, pressing firmly but gently on cake to remove any gaps in layers. Sprinkle with remaining 3 tablespoons rum and spread remaining custard over soaked cake. Cover trifle with plastic wrap and refrigerate for at least 6 hours or up to 2 days.
Using stand mixer fitted with whisk attachment, whip remaining 1 cup cream and remaining 2 teaspoons sugar on medium-low speed until foamy, about 45 seconds. Increase speed to high and whip until soft peaks form, about 1 minute. Spread or pipe cream over top of trifle. Decorate with sprinkles, if using, and serve.